Purpose
This Reference Material can be used to qualify the Spectral Bandwidth, in the ultra-violet region of the spectrum (265 nm – 270 nm), of spectrophotometers with spectral bandwidths of 3 nm or less.
It is accepted for this purpose by most Pharmacopoeias and Standardisation Bodies.
Description and Discussion
This reference comprises two far UV quartz cells, one filledwith a 0.020% v/v solution of toluene in hexane and the othera blank, filled with hexane only. Both have been permanentlysealed by heat fusion.
Resolution is the ability of the instrument to discriminate between two adjacent absorbance bands. This is usually described in terms of the Spectral Bandwidth (SBW) of the instrument, which describes the range of wavelengths coming from the exit slit of the instrument’s monochromator. The accuracy of the measured absorbance will depend on the ratio of the SBW to the natural bandwidth (NBW) of the absorbing substance. The most widely used reference for qualifying the bandwidth of a spectrophotometer is a solution of toluene in hexane.
When scanned in an ultraviolet spectrophotometer, the spectrum shows a maximum absorbance at 269 nm and a minimum at 267 nm.
The ratio of the peak maximum at 269 nm to the minimum at 267 nm gives a measure of the resolution of the instrument. Typical ratios for different bandwidths (at 20°C) are:
| Spectral Bandwidth (nm) | 0.5 | 1.0 | 1.5 | 2.0 | 3.0 |
|
Ratio |
2.4-2.5 | 2.0-2.1 | 1.6-1.7 | 1.3-1.4 | 1.0-1.1 |
Note: The ratio values given in this document are for guidance only. The Calibration Certificate accompanying the Referencematerial gives the actual value measured at a bandwidth of 1.50 nm. Guidance is also given on the values to be expected at different bandwidths (0.5, 1.0, 1.5, 2.0 and 3.0 nm) and temperatures (20, 25 and 30 °C). To comply with Pharmacopoeia requirements, the instrument to be tested should have a spectral bandwidth not exceeding 1.8 nm.
